1. Field of the Invention
The present invention relates to monitoring and controlling quality in a continuous sheetmaking machine, and more particularly, to fast machine and cross direction control of headbox and forming elements of a sheetmaking machine using wet end measurements.
2. State of the Art
In the manufacture of paper using a continuous sheetmaking machine, a web of paper is formed from an aqueous suspension of fibers (stock). Stock is dispersed from a dispensing unit referred to as a headbox onto a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dry section where steam heated dryers and hot air completes the drying process. The sheetmaking machine is essentially a de-watering, i.e., water removal system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction. Furthermore, in general, the elements of the system including the headbox, the web, and those sections just before the dryer are referred to as the xe2x80x9cwet endxe2x80x9d, The xe2x80x9cdry endxe2x80x9d generally includes the sections downstream from the dryer. Papermaking elements and machines are well known in the art and are described, for example, in xe2x80x9cHandbook for Pulp and Paper Technologistsxe2x80x9d 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and xe2x80x9cPulp and Paper Manufacturexe2x80x9d Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking machines are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the art of making paper the sheet properties must be continually monitored and the sheetmaking machine controlled and adjusted to assure sheet quality and to minimize the amount of finished product that is rejected. This control is performed by measuring sheet variables at various stages in the manufacturing process which most often include basis weight, moisture content, and caliper (i.e., thickness) of the sheet, and using this information to adjust various elements within the sheetmaking machine to compensate for variations in the sheetmaking process.
Typically, a scanning sensor is used to perform basis weight measurements of the finished sheet at the dry end of the sheetmaking machine. Scanning sensors are known in the art and are described, for example, in U.S. Pat. Nos. 5,094,535, 4,879,471, 5,315,124, and 5,432,353. The scanning sensor continuously traverses the finished sheet in the cross-direction of the sheetmaking machine. Since the web is moving while the sensor is being scanned, the scanning sensor traverses a diagonal path across the sheet and, as a result, the measured basis weight information provided from the scanning sensor relates to variations in both the machine-direction and cross-direction of the web. The interrelated CD and MD basis weight scanner measurements are further processed and averaged with previous scans to obtain an estimation of independent CD and MD basis weight measurements. Sheetmaking machines are designed with the capability of being independently adjusted to compensate for both CD and MD process variations. The estimated CD and MD basis weight measurements obtained from the scanner are used to control elements in the sheetmaking machine to adjust basis weight in both of these directions.
One of the main disadvantages of scanning sensors is the amount of time that passes from the time that process variations occur in the sheetmaking process to the time the scanning sensor can detect the variations and initiate compensating system adjustments. For instance, the amount of time for stock dispensed from the headbox to travel to the dry end scanning sensor can be lengthy. A typical scan time, (i.e., the amount of time it takes for the scanner to traverse the web) is approximately 16 inches/sec generally resulting in a full sheet scan time of 10-30 seconds. An estimation is obtained by taking 5-8 scans to provide an accurate estimation of the cross and machine direction basis weights. As a result, it can take from 3-15 minutes to obtain CD and MD basis weight measurements using a scanning sensor at the dry end of the sheetmaking machine.
Hence, a sheetmaking machine using a scanning sensor to detect basis weight provides a relatively slow response time to variations in basis weight due to the delay time involved in obtaining basis weight measurements from the scanning sensor. As a result, a sheetmaking machine using a scanning-type sensor is ineffective for detecting rapid variations (i.e., high frequency) in basis weight and particularly variations that occur in the time period less than the amount of time it takes to obtain the basis weight information. In addition, the CD and MD basis weight measurements obtained from the scanning sensor are only an estimation of the actual CD and MD basis weight since the scanning device measurement can only provide interdependent CD/MD basis weight measurements.
What is needed is an manner in which to detect high frequency process variations in an independent manner in both the machine and cross directions and use these detected variations to independently adjust MD and CD controllable elements in the system.
The present invention is a system and method for detecting high frequency variations in basis weight at the wet end of a sheetmaking machine and providing on-line control to elements in the system to compensate for the detected variations. The sheetmaking machine is designed with non-scanning sensors which provide simultaneous multiple point wet end, water weight measurements across either/or both of the machine direction (MD) and cross direction (CD) of the sheetmaking machine. These water weight measurements are converted into predicted dry end basis weight measurements. The predicted basis weight measurements are then used to make quick system adjustments to elements in the sheetmaking machine to compensate for process variations. The non-scanning sensors obtain independent MD and the CD water weight measurements and hence can independently monitor predicted basis weight of the dry sheet in each of the cross and machine directions. In addition, the non-scanning sensors are situated in the wet end of the sheetmaking machine thereby providing quick basis weight readings.
The predicted dry end basis weight information is provided to at least one system controller which, in response, provides on-line control signals for adjusting operating variables of sheetmaking machine elements. In a particular embodiment, the wet end sensors are under wire water weight (UW3) sensors which are responsive to changes in conductivity of the aqueous stock material at the wet end of the system. In one embodiment, operating variables that can be adjusted by the on-line control signals include headbox pressure, headbox flow, headbox total diluation, jet-to-wire ration, forming board machine direction position relative to the wetstock impingement region, and forming board angular position relative to the wire. On-line control signals can also be used to control wetstock source elements which feed wetstock to the headbox.
In another embodiment, a sheetmaking machine includes first and second control loops for controlling operating variables of machine elements in the sheetmaking machine to compensate for process variations. The first control loop includes a non-scanning wet end measurement sensor for obtaining independent wet end basis weight measurements in both the MD and CD, a dry end basis weight predictor for converting the independent wet end basis weight measurements in both the MD and CD into predicted independent dry end basis weight measurements in both the MD and CD, and a first controller responsive to the predicted independent dry end basis weight measurements. The first control loop has a relatively quick response time and hence can compensate for high frequency basis weight variations due to its proximity to the system elements being controlled (e.g., headbox and forming elements) and the wet end sensor response. The second control loop includes a dry end measurement sensor and a second controller responsive to the dry end sensor measurements. The second control loop has a slower response time relative to the first control loop since the dry end measurement sensor resides farther down the sheetmaking machine processing path. The second loop compensates for larger basis weight variations so as to keep end product basis weight within a set range. In one embodiment, the first and second controllers adjust operating variables by controlling various aspects of wetstock source, headbox and forming elements of the sheetmaking machine and in particular provide on-line control for controlling headbox pressure, headbox flow, headbox total dilution flow, headbox air pad, jet-to-wire ratio, forming board machine direction location, and forming board angle relative to wire, and refiner loading.